Classical molecular-dynamics potential for the mechanical strength of nanocrystalline composite fccAl+α−Fe2O3

Abstract

A classical molecular-dynamics potential for analyzing mechanical deformation in the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}+\mathrm{fcc}\text{\ensuremath{-}}\mathrm{Al}$ material system is developed. The potential includes an embedded atom method cluster functional, a Morse-type pair function, and a second-order electrostatic interaction function. It is fitted to the lattice constants, elastic constants, and cohesive energies of fcc Al, bcc Fe, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, and B2-FeAl, accounting for the fact that mixtures of Al and ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ are chemically reactive and deformation may cause the formation of these components as reaction products or intermediates. To obtain close approximations of the behavior of mixtures with any combination of the atomic elements, the potential is formulated and fitted such that the Al-Al, Fe-Fe, Al-Fe, O-O, Fe-O, and Al-O interactions are accounted for in an explicit and interdependent manner. In addition to being fitted to the lattice constants, elastic constants, and cohesive energies, the potential gives predictions of the surface and stacking fault energies for the crystalline components that compare well with the predictions of established potentials in the literature for the corresponding crystalline components. The potential is applied to analyze quasistatic tensile deformation in nanocrystalline Al, in nanocrystalline ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, and in nanocrystalline $\mathrm{Al}+{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ composites. Application of the potential to nanocrystalline Al reveals the features of mechanical deformation, such as the formation of unit dislocations, flow strength approaching ideal shear strength, and the Hall-Petch relationships, that are in close agreement with experiments and with the predictions of established potentials for Al in the literature. Analyses of deformation in nanocrystalline ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and in nanocrystalline $\mathrm{Al}+{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ composites point to the possibility that the strength of the nanocomposites can only be calculated using the mixture theory if the average grain size is above a critical value. Below the critical grain size, an accurate account of interfacial stresses is important to the prediction of the strength. For composites with grain sizes above the critical value, the observed dependence of strength on volume fraction is in agreement with experimental observations.